A Legionnaires’ disease outbreak in Basel, Switzerland, has claimed one life and sickened 26 people, with health authorities investigating the source of the bacteria responsible for the potentially fatal form of pneumonia.

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An outbreak of Legionnaires’ disease in Basel caused 26 people to fall ill, one fatally, the Swiss city said Monday, with an office block’s cooling towers thought to have been the cause.

“The medical services of the health department are currently observing an increased incidence of Legionnaires’ disease,” the Basel-City canton said in a statement.

It said 26 cases had been confirmed, of whom 25 were hospitalised, including seven needing intensive care.

Nine people remain in hospital — including one in intensive care — and “sadly, one person has died,” the canton said.

Legionnaires’ disease is caused by exposure to the legionella bacterium found in water and soil. It ranges in severity from a mild influenza-like illness to a serious and sometimes fatal form of pneumonia.

Symptoms include fever, headache, lethargy, muscle pain, diarrhoea and sometimes coughing up blood. The disease can be treated with antibiotics.

The disease does not spread directly between humans. Outbreaks are often linked to poorly maintained air conditioning and water systems, humidifiers and whirlpool spas.

Most of the cases occurred in the same area of central Basel, close to the River Rhine, though isolated cases flared up in other parts of the northern city, which sits right on the border with France and Germany.

“The cluster suggests one or more common sources of infection,” the canton said, with investigations focused on water cooling towers.

After samples were taken from 10 cooling facilities, high levels of legionella pneumophila — responsible for most human infections — were detected at two facilities on the roof of an office building.

They were shut down onFriday,y and both are being refurbished.

The Basel health authorities are continuing monitoring and inspection, while residents with pneumonia symptoms such as fever and shortness of breath should consult their doctor.

With a population of 180,000, Switzerland’s third-biggest city after Zurich and Geneva is one of Europe’s great centres of culture, though the chemical and pharmaceutical industries now drive Basel’s economy.